A zeolite has a peculiar three-dimensional structure of alumino-silicate and has a large micropore and an excellent ion exchanging property as compared to another alumino-silicate crystal, thus being extensively used as a catalyst, an adsorbent, a molecular sieve, and an ion exchanger. The purpose of a natural zeolite is limited due to a structural limitation thereof, but the purpose of a synthetic zeolite is gradually expanding. In order to diversify the purpose of the zeolite, an economical synthesis method is required, and a crystal size, a particle size distribution, and a shape of the zeolite need to be arbitrarily adjusted.
A high silica zeolite is a zeolite having a high ratio of silica to alumina, and in ZSM-5 as one of the zeolites, a three-dimensional pore having 10-tetrahedron rings is formed, and a size thereof is approximately an intermediate of those of zeolite A, zeolite X, and zeolite Y. Further, ZSM-5 is a kind of pentasil zeolite as a shape-selective catalyst having peculiar adsorption and diffusion properties, in which a SiO2/Al2O3 ratio is high and thus, generally, thermal stability is good, there is hydrophobicity, a Lewis acid site is large, and a Bronsted acid site is small. Particularly, gasoline fractions having a high octane number may be directly obtained from methanol by a MTG process, and ZSM-5 is known to have excellent selectivity to the gasoline fractions.
Since first development of ZSM-5 having a high silica content by Mobil company in the early 1970s, many studies of this material have been conducted due to peculiar catalytic activity and shape selectivity caused by a molecular sieve effect of this material. Unlike a general aluminol-silicate zeolite, various kinds of organic materials have been used as a structure inducing material for forming a structure to manufacture ZSM-5.
Until now, among the organic materials known to be effective to form the structure of ZSM-5, tetrapropyl ammonium cations are known to have the best structure inducing effect, and ZSM-5 that has been come onto the market in recent years is mainly synthesized by using the aforementioned material. However, even though the organic structure inducing materials including the tetrapropyl ammonium ions have the excellent structure inducing effect, since the organic structure inducing materials are disadvantageous in terms of economic and environmental aspects, studies have been conducted to exclude use of the organic structure inducing materials, and some processes relating to the studies have been developed. The reason why the organic structure inducing materials are excluded is that the materials are very expensive and have very strong toxicity, thus causing environmental pollution. Further, in the case where ZSM-5 is synthesized by using the organic structure inducing materials, secondary costs are required to treat toxic unreacted organic structure inducing materials contained in waste water.
Further, the structure inducing material contained in the manufactured ZSM-5 crystal particles should be thermally decomposed to be removed through calcination at 550° C. or more before used, and in the case where thermal decomposition does not completely occur during a removal process by calcination, blocking of pores may occur to cause fatal flaws to catalytic activity. Further, an additional burden of expense according to calcination and air pollution due to discharge gas generated during thermal decomposition of the organic materials are unavoidable.
Accordingly, in order to overcome the aforementioned limitations, Flanigen et al reported for the first time a method of synthesizing ZSM-5 using or not using a crystalline nucleus with exclusion of an organic structure inducing material. However, in the aforementioned method, a reaction time is very long 68 to 120 hours. Further, in the case where ZSM-5 is synthesized with exclusion of the organic structure inducing material, since synthesis is very sensitively affected by reaction conditions, meticulous care is required.
Meanwhile, examples of factors affecting synthesis of the high silica zeolite may include a type of a silica source, a Si/Al ratio, a concentration of an alkali solution, the mixing order of reactants, a crystallization temperature, a crystallization time, the degree of aging, and presence of agitation. Among the various factors, the type of the silica source is known as the most important factor.
According to Eastern Germany Patent No. 207185, generally, sodium silicate and a silica sol are used as a silica source. Sodium silicate is a type where water is added to solid silicate (cullet) to perform dissolving and is cheapest among the silica sources, but contains an alkali component in a great amount, and thus there is difficulty in controlling of a reactant composition, and a sulfuric acid or aluminum sulfate should be added to control an alkali concentration in sodium silicate.
Further, since a reaction condition is complicated, the zeolite is non-uniformly crystallized, and a cost of post-treatment such as removal of a metal salt is high.
According to Eastern Germany Patent No. 207186, in the case where a silica sol is used as a silica source, even though the silica sol has good reactivity and is easily treated, a raw material cost is high as compared to another silica source, and a silica component is finely dispersed in water in a great amount in a colloidal state and is rapidly reacted with an alumina component to generate a hydrogel, and thus in order to prevent this, the two components should come into contact with each other in a dilution state. In this case, since a solid content is low based on particles crystallized during a zeolite synthesis process and zeolite crystal particles are finely dispersed in a unit particle state, many loads occur during filtrate separation and water-washing processes, unreacted components are discharged while being contained in a great amount in the filtrate and a water-washing solution, and thus, resultantly, unit productivity is low, accordingly, there is a limitation as an industrial production method.
In addition, Korean Patent Application Laid-Open No. 10-2007-0020354 discloses a method of manufacturing a zeolite molecular sieve catalyst having a small crystal size by using diatomite or silica aerogel as a main silica source, adding a seed determining alignment agent, a silica sol, and sodium silicate for kneading and shaping, and performing gas-solid crystallization by organic amine and steam to perform conversion into an integrated zeolite having the small crystal size. However, in the aforementioned method, a nano-sized seed and organic amine are used to obtain the zeolite having fine particles, thus increasing a process cost.
As described above, according to the method of synthesizing the high silica zeolite reported until now, in the case where the high silica zeolite is synthesized using low-priced sodium silicate as the silica source by the method of excluding the organic structure inducing material, a chemical composition of a reactant capable of synthesizing the zeolite having high crystallinity is limited, and the zeolite having a long crystallization time and low particle uniformity is synthesized. Further, in the case where the high silica zeolite is synthesized during a practical process, since about 40 to 70 wt % of the silica used as the raw material is present in an unreacted state in the filtrate, production yield of the zeolite is low and a post-treatment cost is high due to generation of a great amount of waste water. Therefore, there is a demand for developing a zeolite synthesis technology of recovering a great amount of unreacted silica generated during synthesis of the high silica zeolite to be reused and thus prevent a waste of a synthesis raw material and minimize generation of waste water.
Ind. Eng. Chem. Res. (vol. 49, 7294 (2010)) reports that when a high silica zeolite is synthesized, even though a filtrate containing an unreacted silica is recovered to be reused during synthesis of the zeolite, there is no problem in synthesis. However, in the case where a metal salt generated during synthesis is not removed from the filtrate but repeatedly reused, since the metal salt is accumulated in a mother liquid to affect synthesis, there is a limitation in reuse and recovery, and thus the metal salt generated when the zeolite is synthesized should be removed to recover the unreacted silica from the filtrate and repeatedly reuse the silica.
Meanwhile, methods of manufacturing the silica sol from sodium silicate have been reported. U.S. Pat. No. 2,605,228 proposes a method of manufacturing a silica sol by diluting a sodium silicate solution having a mole ratio of SiO2/Na2O of 3.2/1 by water, performing acid treatment by a sulfuric acid or a hydrochloric acid, performing heat treatment at 60 to 100° C. to grow particles, adding divalent cations to perform agglomeration, and peptizing a precipitated silica cake by an alkali through filtering, washing, and cation exchanging processes. Further, Japanese Patent Application Laid-Open No. 63-285112 describes a method of manufacturing a silica sol, which includes treating an alkali metal silicate aqueous solution by a strong cation exchange resin in order to obtain a colloidal solution of active silica, adjusting a pH of silicate from 0 to 2, treating the solution by a strong acidic cation exchange resin, treating the solution by a basic anion exchange resin, and treating the solution by the strong acidic cation exchange resin.
As described above, in the existing methods of manufacturing the silica sol from sodium silicate, since a complicated process of manufacturing the silica sol is performed through a process of manufacturing the sodium silicate solution, an ion exchange process using the ion exchange resin, and agglomeration and peptizing processes using the divalent ions, a high treatment cost and a great amount of waste water are required during deoxidation and washing processes by an acid reaction or a regeneration process of the ion exchange resin after the ion exchange reaction.
Further, since a great amount of metal salt as well as the unreacted silica is contained as an impurity in the filtrate generated during synthesis of the zeolite, in the case where the method of manufacturing the sol from pure sodium silicate is applied to manufacture the silica sol from the filtrate containing the metal salt, agglomeration is severe, and thus it is difficult to perform solation.
Accordingly, the inventors of the present invention found that when a method of manufacturing a high silica zeolite, in which a silica sol is manufactured from a recovered silica filtrate to be reused as a silica source during a process of synthesizing the high silica zeolite, is used, since a cation exchange process is omitted due to an oxyanion effect during a process of converting the silica filtrate into the silica sol, a manufacturing process is simplified, a metal salt included as an impurity during a solation process is removed to prevent agglomeration of the silica sol and thus control a particle size of the silica sol, a process energy cost, a chemical treatment cost, and a waste water treatment cost are reduced, and a manufacturing time is shortened, thereby accomplishing the present invention.